This updated Medicines Q&A considers the factors which need to be considered when converting patients from oral morphine to fentanyl patches.
- Explicit guidance on switching opioids is difficult because both the reasons for switching and the patient’s circumstances differ.
- When initiating transdermal fentanyl in a patient currently treated with oral morphine, the initial dose should be based upon their previous 24-hour opioid requirement.
- Two parallel dosage conversion ratios are currently recommended by most manufacturers of fentanyl patches.
- Data from two small studies has shown that conversion of oral morphine to transdermal fentanyl with a ratio of 100:1 is safe and effective. This ratio is endorsed by the Palliative Care Formulary, and is also advised by the British National Formulary for patients who have been on a stable and well-tolerated opioid regimen for a long period and who need opioid rotation.
- An alternative ratio of oral morphine to transdermal fentanyl of 150:1 is recommended by the PHE Opioids Aware Resource, and is advised by manufacturers for patients who have been stabilised on oral morphine for several weeks and who need opioid rotation.
- The decision regarding which conversion ratio to use should be based on patient specific factors, the level of patient monitoring which can be offered and the clinical setting.
- Comparative doses of oral morphine and transdermal fentanyl are given in Tables 1 and 2. These should be used in conjunction with the Summary of Product Characteristics for the fentanyl patch being used.
- If switching because of possible opioid-induced hyperalgesia, the calculated equivalent dose should be reduced.
- Patients who are taking a daily dose of morphine that is not exactly equivalent to a particular fentanyl patch strength, will need to be changed to a patch which is either slightly less or slightly more potent than the morphine dose. This will be a clinical decision based on patient specific factors and patients should be monitored.
- The CQC and NHS England advised to: “formally double check the calculations and where possible have the patient’s dose independently verified”
- Effective systemic analgesic concentrations of fentanyl are reached less than 12 hours after applying the patch. Guidance is given above regarding the discontinuation of the regular oral morphine dose.
- Serum fentanyl concentrations increase gradually and level off between 12 and 24 hours. Steady state concentrations are reached by the second 72-hour application. The patient should use breakthrough analgesia as required for the first 3 days particularly the first 24 hours.
- Patients will require frequent reassessment of efficacy and adverse effects and dose titration after patch initiation or dose change as well as a short acting opioid for breakthrough pain.
- The dose of laxative(s) may need to be reduced and subsequently adjusted according to need as fentanyl is less constipating than morphine.
Medicines Information Pharmacist, South West Medicines Information and Training